Method and Device for Plasma-Assisted Chemical Vapour Deposition on the Inner Wall of a Hollow Body

ABSTRACT

The invention relates to a method for plasma-assisted chemical vapour deposition for coating or material removal on the inner wall of a hollow body ( 42 ). The method involves introducing a gas lance ( 44 ) into the hollow body ( 42 ) and forming a cavity plasma ( 45 ) to form a plasma cloud arranged at the tip of the gas lance by applying an electric radio-frequency field to an RF electrode ( 41 ).

The invention relates to a method for plasma-assisted chemical vapourdeposition for coating or material removal on the inner wall of a hollowbody.

Such methods are known by the generic terms plasma coating (PECVD,“Plasma Enhanced Chemical Vapour Deposition”) or ion etching and plasmaetching.

In this context, a workpiece is introduced into a vacuum chamber andfixed there. The chamber is evacuated to a residual gas pressure in thehigh-vacuum or ultra high-vacuum range and an inert working gas isadmitted. A low-pressure plasma is subsequently ignited by feeding in anRF field via an RF electrode arranged in the vacuum chamber. An ionizedgas is produced in this case, said gas containing an appreciableproportion of rapidly moving free charge carriers such as ions orelectrons.

In PECVD, besides the working gas further reaction gases, as they arecalled, are fed into the chamber, which gases can be, in particular,carbon-containing or silicon-containing gases. In the low-pressureplasma, the electrons have such high energies that chemical reactionsbetween the gas constituents and constituents of the surface of theworkpiece are possible which are not possible at thermal equilibrium.Layers form on the surface of the workpiece in this way, which layerscan comprise carbon or silicon oxide, for example, depending on thereaction gas. It is thus possible for example to produce high-strength,low-friction and biocompatible diamond-like carbon (DLC) coatings whichare used e.g. in implants, gearwheels and the like.

Ion etching and plasma etching, by contrast, involve removing materialfrom the surface of the workpiece in order e.g. to clean the latter. Forthis purpose, the ions of the low-pressure plasma generated have to havea certain minimum energy. The acceleration of argon ions in the highvacuum in the direction of the substrate to be processed has the effectthat, upon impingement, momentum is transferred from the high-energyions to the substrate and the surface of the latter is sputtered andremoved uniformly.

In plasma etching, moreover, the etching is effected by a chemicalreaction. In this case, instead of pure argon a reactive gas such asoxygen, for example, is fed to the plasma.

Both PECVD and also ion etching and plasma etching have proved to beextremely worthwhile in the surface treatment of workpieces. However, atleast when a plasma is generated by radio-frequency excitation, neitherof these methods is suitable for coating or for etching the innersurfaces of hollow bodies such as e.g. containers, bottles, tubes,cannulas, bores and the like.

This is due to the fact that conductive hollow bodies form a Faradaycage in the electric field. The ions produced are oriented according tothe field lines of said electric field. Since these run along and aroundthe outer wall of the hollow body but not through its inner volume, aninner coating is not physically possible in a straightforward manner. Inorder to circumvent this effect, the plasma has to be brought into theinner volume of the hollow body. In this case, a negative area replacingthe role of the inner wall of the chamber as negative area would have tobe introduced into the inner volume. In this case, the size of thenegative area must in principle be at least twice as large as thesurface to be coated, in order to ensure a deposition sufficient for thelayer construction.

Accordingly, it is practically not possible to comply with thisprinciple within a hollow body.

In the case of a cylindrical hollow body, by way of example, the innersurface area of the cylinder wall is A=2 πrh. A planar electrode, set upperpendicularly in the hollow body, could have at most a surface area of2 rh, however, that is to say would be smaller than the surface to becoated by a factor of 3.14 rather than being twice as large according tothe technical requirement.

Similar relationships hold true for other hollow bodies such as e.g.cones and truncated cones or complexly shaped hollow bodies.

DE 197 26 443 describes a method for surface refinement of innersurfaces of hollow bodies in which the plasma is ignited by a hollowcathode corona discharge. What is disadvantageous here is that onlyrelatively short hollow bodies in which the depth does not exceed theopening diameter can be coated from the inside. A variant that enableslonger hollow bodies to be coated on the inside provides for the hollowcathode to be inserted into the hollow body and to run along the innerside. Thus, although longer hollow bodies can be coated on the inside,they nevertheless have to have a rectilinear wall course.

EP 1 548 149 describes a method for forming a thin oxide coating on theinner side of a hollow body. In this case, a hollow body to be coated onthe inner side is introduced into a cylindrical chamber that functionsas an RF electrode. A gas tube, which simultaneously functions as anearth electrode, is introduced into the interior of the hollow body.

The disadvantage of this method resides in the formation of the layerproperties. The gas tube functions as an earth electrode in the devicedescribed in EP 1 548 149. For this reason, the layer properties(hardness, thickness, lattice structure of the deposition, purity of thelayer, doping with functional elements, water-repelling or -absorbing)cannot be established as desired.

It is not possible to establish and control these properties in the caseof an introduced earth electrode which, with respect to its area, issmaller by a factor of 1 than the area to be coated.

DE3821815 discloses a device for coating an inner wall of a hollow bodywith a diamond-like hard carbon coating with the aid of aplasma-assisted CVD method. In this case, a process gas containing atleast one hydrocarbon gas is conducted through the interior of theunheated hollow body, in which a plasma excites the process gas, whereinit is dissociated and ionized and the resulting ions, for forming thecoating, are accelerated onto the inner wall to be coated. The devicehas an RF generator connected to the hollow body, with an earthingarrangement for forming the plasma between the hollow body and theearthing arrangement and with a feed line opening into the interior ofthe hollow body for the controlled introduction of the process gas intothe interior of the hollow body. The earthing arrangement is connectedto a vacuum housing into which the interior of the hollow body leads andwhich surrounds the hollow body at a distance from the lead-in junction.

This device has proved to be unsuitable in practice, for variousreasons. Thus, in the method carried out with this device, not only theinner wall of said hollow body but also the outer wall thereof iscoated. Furthermore, this device is only suitable for the coating ofhollow bodies having a rectilinear inner course (so-called “blindholes”), that is to say e.g. not for container-like vessels having anarrowed neck.

An additional factor is that in this device the hollow body itselffunctions as an electrode since it is conductively connected to theradio-frequency electrode. This is necessary in this device sinceotherwise the field strength of the electromagnetic alternating fieldgenerated would not suffice to ensure an inner coating. The penetrationdepth of an electromagnetic alternating field generated only by theradio-frequency electrode in the base region of the vacuum chamber (thatis to say the maximum thickness of an if appropriate metallic materialthrough which the alternating field penetrates with sufficient strengthstill to initiate a coating reaction) is in the region of 2 cm in thisdevice. Therefore, in this device, hollow bodies having a larger wallthickness have to rely on the fact that they themselves function as anelectrode; therefore, they must necessarily be composed of metal.

Furthermore, it has been found that the geometries of the hollow bodiesto be coated are very limited. Thus, despite describing that besidesworkpieces having a ratio of tube diameter to tube length in the rangeof between 20 mm to 60 mm and 2 mm to 20 mm, tube diameters of greaterthan 20 mm and less than 2 mm, respectively, can also be coated withthis device, this has been found to be problematic in practice.

Therefore, the abovementioned method is not suitable for a large numberof applications in which hollow bodies having a larger internal diameterare intended to be coated.

It is an object of the present invention to provide a method forplasma-assisted chemical vapour deposition for coating or materialremoval on the inner wall of a hollow body which does not have thedisadvantages mentioned.

This object is achieved by means of a method having the features ofnewly presented claim 1. The dependent claims specify preferredembodiments.

It should be taken into account here that value ranges delimited bynumerical values should always be understood as inclusive of therelevant numerical values.

Accordingly, provision is made of a method for plasma-assisted chemicalvapour deposition for coating or material removal on the inner wall of ahollow body, in particular composed of a non-metallic material, having across-sectional area, a longitudinal extent and at least one opening.The method has the following steps:

1. introducing the hollow body to be coated on its inner side into avacuum chamber with an earthed inner side, a large-area radio-frequencyelectrode being arranged in the interior of the vacuum chamber,2. positioning the hollow body in the centre of the vacuum chamber, itbeing necessary to comply with a minimum distance of 15 cm on all sidesbetween the outer wall of the hollow body and the inner wall of thevacuum chamber,3. introducing a gas lance comprising a tube having an internal diameterof 0.001-10 mm, a maximum external diameter of 12 mm and a terminalnozzle having a terminal opening diameter of 0.002-6 mm through theopening into the hollow body, the gas lance being connected to a gasfeed unit via a non-electrically conductive line and, in particular, notbeing earthed or being in electrically conductive contact with theradio-frequency electrode,4. positioning the gas lance in the hollow body in such a way that thegas lance is positioned centrally relative to the cross section of thehollow body and the nozzle of the gas lance, relative to thelongitudinal extent of the hollow vessel, is arranged in the region ofthe transition from the second longitudinal third to the thirdlongitudinal third, measured from the opening of the hollow body,5. closing the vacuum chamber and evacuating the latter to a residualpressure of 0.001-5 pascals,6. introducing an inert working gas and also one or a plurality ofreaction gases via the gas feed unit and the gas lance into the hollowbody, and7. igniting a cavity plasma to form a plasma cloud arranged at the tipof the gas lance, by applying an electric radio-frequency field to theRF electrode.

It is relevant here that the hollow body to be coated is not earthed. Itis preferably provided here that the vacuum chamber is evacuated to aresidual pressure of 0.01-2 pascals. Particularly preferably, the vacuumchamber is evacuated to a residual pressure of 0.1-1 pascal.

What is important in this method is that the gas lance is not groundedor earthed, but rather is electrically insulated. For this purpose, itis preferably provided that said gas lance is insulated with the aid ofa PTFE ring (polytetrafluoroethylene) and the gas supply line within thechamber interior is produced from PTFE. Suitable hollow bodies to becoated include, in principle, all possible hollow bodies, that is to saynot only hollow bodies closed on one side (such as e.g. vessels,containers, etc.) but also tubular hollow bodies without a base, such ase.g. cannulas, bodies having a through hole or tubes. The latter hollowbodies have to be closed off with a cover or stopper on one side priorto coating.

In both cases care must be taken to ensure that the gas lance isarranged in the hollow body in such a way that the gas lance ispositioned centrally relative to the cross section of the hollow bodyand the nozzle of the gas lance, relative to the longitudinal extent ofthe hollow vessel, is arranged in the region of the transition from thesecond longitudinal third to the third longitudinal third, measured fromthe opening of the hollow body. This means that the gas lance has to beadvanced to relatively just before the bottom of the vessel (or beforethe second opening of the hollow body closed off with a cover orstopper). A minimum distance of 10 cm has to be complied with here. Inthe case of substrate objects having a depth of 10 cm or less, the tipof the gas lance is positioned directly above the opening of the hollowbody.

In principle, low-pressure plasmas as in the present invention ensure alarger mean free path length λ of the gas molecules and therefore delaythe formation of a plasma. The arrangement of the gas lance according tothe invention has the effect, by contrast, that the gas moleculesemerging from the gas lance collide with the bottom of the vessel or theabovementioned cover or stopper as a result of their acceleration. Thispromotes the gas dissociation process and the formation of a plasma. Forthis reason, a comparatively lower strength of the electromagneticalternating field is sufficient, that is to say that it is not necessaryfor the hollow body that is to be coated itself to function as anelectrode.

Preferably, the minimum distance between the outer wall of the hollowbody and the inner wall of the vacuum chamber is 15 cm. The maximumdistance, by contrast, is given by the dimensioning of the vacuumchamber used.

The gas lance has preferably an internal diameter of 0.005-6 mm andparticularly preferably an internal diameter of 0.01-6 mm or 0.1-6 mmand a maximum external diameter of 10 or 8 mm. The terminal nozzlepreferably has a terminal opening diameter of 0.01-3 or 0.1 to 2 mm.

The arrangement and dimensioning of the gas lance ensure that the plasmaforms only at the tip of the gas lance, i.e. only in the interior of thehollow body to be coated. Since the gas molecules experience theirgreatest acceleration at the instant of the plasma-induced dissociation,this acceleration fully benefits the treatment of the inner surface ofthe hollow body. Therefore, it is also possible to dispense with anelectrode in the interior of the hollow body.

In this way, a plasma is enabled to be ignited and maintained only inthe interior of the hollow body. This type of plasma is referred to as“cavity plasma” hereinafter. This ensures that said hollow body iscoated only on the inner side and not on its outer side.

The plasma-induced molecular dissociation takes place at the instant atwhich the gas mixture leaves the lance nozzle. It takes place with theformation of very short-wave light.

The splitting energy released during dissociation accelerates the whatis now truly “plasma matter” to approximately 250,000 km/h. On accountof this acceleration, the carbon impinges on the inner surface to becoated and deposits as a hard material layer. The type of depositionvaries depending on the gas used and the purity and composition thereof.

The dissociation ratio is 1:12 e.g. in the case of H₂C₂. This means thatthe H atoms are 12 times lighter than the C atoms. The acceleration ofthe dissociation plus the acceleration of the individual atoms and theimpingement on the substrate are therefore in the ratio of 1:12.

Therefore, twelve times more C atoms than H atoms at the same velocityimpinge on an identical area in the same period of time. Since the Hatoms are undesirable in a hard material layer, the quantity of reactiongas fed in has to be calculated with regard to the inner area to becoated.

The following relationship determined empirically by the inventor can beused for calculating the quantity of reaction gas to be fed in:

V=A/12*E

In this case, A is the surface area to be coated [cm²], E is thedissociation energy supplied, and V is the volume of the reaction gasper minute [cm³/min].

On account of the mass inertia and the dissociation energy released, thecarbon therefore requires less area per acceleration free space in orderto arrive at the required 250,000 km/h.

If the H₂C₂ is introduced in a three-dimensional hollow body by means ofa gas lance, then it must be ensured that the C atoms impinge directlyon the substrate at maximum acceleration and are not deflected,decelerated or even stopped by equivalently accelerated H atoms.

This is ensured by virtue of the fact that the nozzle of the gas lance,relative to the longitudinal extent of the hollow vessel, is arranged inthe region of the transition from the second longitudinal third to thethird longitudinal third, measured from the opening of the hollow body.As a result, the atoms are accelerated up to their maximum and impingeon the substrate directly at the end of this phase without being impededby other atoms.

Investigations by the inventor have revealed moreover, that in order toensure the abovementioned deposition on the inner surface of a hollowbody, the dissociation energy (E_(A)) in watts must be higher than theopening diameter (D_(Ö)) of the hollow body in cm at least by the factor65.5.

This means, therefore, that given an opening diameter (D_(Ö)) of thehollow body of 15 cm, the dissociation energy (E_(A)) on the basis ofthe relationship

E _(A) =D _(Ö)*65.5

must be at least 15*65.5=982 watts.

By introducing this minimum dissociation energy, which can becorrespondingly established at the RF generator, the atoms of thereaction gas that has undergone transition to the plasma are acceleratedin such a way that their oscillation amplitude is larger than theopening diameter of the hollow body. This ensures that only transverselyaccelerated atoms can leave the hollow body.

In this way, contrary to the statements in the introduction, the innersurface of a hollow body can also be coated by means of the methodaccording to the invention.

In this case, the dimensioning of the nozzle of the gas lance preventsthe plasma from flashing back into the gas lance as would be feared withnozzles having larger dimensions.

It is also important that the diameter of the gas lance does not widenin the direction of the nozzle. As a result of this, on account of theBernoulli effect, the pressure of the incoming gas would decrease in theflow direction in the region of the cross-sectional widening, whichwould promote a flashback of the plasma into the gas lance and thusdestruction of the gas lance. The formation of the plasma cloud at thetip of the gas lance would be prevented in this way.

In one preferred configuration of the method according to the invention,it is provided that the radio-frequency electrode in the interior of thevacuum chamber has at least two leads via which radio-frequency voltagescan be fed into the radio-frequency electrode.

In this way, an alternating field having very high field strengths suchas is required for forming the cavity plasma can be generated in thechamber. An alternating field generated in this way has a sufficientlyhigh penetration depth, such that even hollow bodies having large wallthicknesses can be penetrated and coated on the inner side. The hollowbody itself therefore does not have to function as an electrode and cantherefore also be composed of a non-metallic material. It is thereforeirrelevant whether the hollow body is in electrically conductive contactwith the radio-frequency electrode or whether it is completelyelectrically insulated.

This feature is advantageous particularly with the property that in themethod according to the invention the temperatures in the interior ofthe coating chamber generally do not exceed 200° C. On account of theselow temperatures, therefore, even plastic hollow bodies can be providedwith an extremely durable inner coating. This is advantageousparticularly because, on account of the unrequired electricallyconductive connection between the hollow body and the radio-frequencyelectrode, non-metallic hollow bodies can indeed also be coated by themethod according to the invention.

In this case, three or more leads are preferably provided since an evenmore homogeneous alternating field can be established in this way.

In this case, it is preferably provided that the individual leads to theradio-frequency electrode are adjusted separately in such a way that ahomogeneous alternating field having uniformly high field strengths canbe generated in the entire chamber. This feature benefits the coatingquality.

This can be effected by means of a so-called matchbox, for example,which is connected between a radio-frequency generator and theradio-frequency electrode. This has e.g. trimming potentiometers for theindividual leads to the radio-frequency electrode which are adjustedseparately. In this case, the same bias voltage is set at all theregulators, which indicates identical field strengths and thus ahomogeneous alternating field.

In a further preferred configuration of the method according to theinvention it is provided that said hollow body merely has an openingwhose narrowest diameter is narrower than the narrowest diameter of theinner space of the hollow body. Such a hollow body can be e.g. a bulkcontainer, a bottle or the like. Hollow bodies having such geometriescannot be coated in particular by the method known from DE3821815.

Furthermore, it is preferably provided that said hollow body to becoated has an inner volume in the range of between a few cm³ and1,000,000 cm³. For technical reasons, a limit is imposed on the size ofthe hollow body to be coated only because the size of the vacuumchambers currently available is limited.

Thus, e.g. a bulk container has an inner volume in the range of10,000-100,000 cm³. An engine block having four cylinders has e.g. fourinner volumes in the range of between 250 and 700 cm³. A gas cylinderhas e.g. an inner volume in the range of 20,000-100,000 cm³.

Here, too, it holds true that hollow bodies having such volumes cannotbe coated with sufficient quality in particular by the method known fromDE3821815.

It is preferably provided that the working gas is a gas selected fromthe group containing argon, helium, hydrogen, oxygen or some other noblegas.

Furthermore, it is particularly preferably provided that the reactiongas is a gas selected from the group containing oxygen.

Such a method for plasma-assisted chemical vapour deposition formaterial removal is also referred to as plasma etching. Oxygen isparticularly suitable as a reaction gas for this method since the oxygenions produced in the plasma are particularly heavy and therefore bringabout surface removal particularly effectively in the accelerated state.

Investigations by the applicants have revealed that e.g. the innersurface of a used bulk container such as is used e.g. for the productionof vaccines and which is extremely contaminated after use by dried-inand/or chemical blood constituents can be cleaned extremely thoroughlyby this method.

Pursuant to applicable regulations, e.g. a high-grade steel for medicaluse must be absolutely free of residues of previous substances incontact with it. This has been achieved hitherto in the case of bulkcontainers, for example, by means of a very costly cleaning processusing acids and alkaline solutions.

The method according to the invention, in which a plasma is ignitedafter oxygen has been fed with supply of high energy, makes it possibleto clean (“etch”) the surface of the substrate in a manner absolutelyfree of residues. This can be attributed in particular to the highatomic weight of the oxygen atoms, which reliably remove contaminantsupon sufficient acceleration.

In a further preferred configuration of the method according to theinvention, it is provided that the reaction gas is a gas selected fromthe group containing hydrocarbon gases such as methane, ethane, ethene,ethyne, propane or silane gases such as tetramethylsilane orhexamethyldisiloxane.

The former reaction gases are suitable for forming a DLC layer and thelatter e.g. suitable for forming an SiO₂ layer.

The term DLC (“Diamond-Like Carbons”) is understood to mean layers ofmolecular carbon which have a net or lattice of sp²- and sp³-hybridizedcarbon atoms. The ratio of the two variants to one another depends onthe coating conditions. If the former predominate, the coating hasgraphite-like properties (low coefficient of friction), and if thelatter predominate, the hardness and the transparency of the coatingincrease. Mixed coatings containing both variants frequently combineboth advantages.

Investigations by the applicants have revealed that the inner surfacesof bulk containers and other hollow bodies can be effectively coatedwith a DLC layer by this method.

Preferably, in the method according to the invention, the plasma isignited by applying a DC voltage radio-frequency field with thefollowing parameters:

1. frequency: 10 kHz-100 GHz2. electrical power: 500-5000 W3. gas feed: 0-90 scm³.

The frequency preferably lies in the range of 10-15 MHz. The frequencyis particularly preferably 13.56 MHz (RF, radio-frequency).

The electrical power to be introduced is calculated according to thefollowing formula: power (watts)=area to be coated (m²)×1750. In thiscase, the factor mentioned last can lie between 1500 and 2200 and isdetermined empirically in practice. A hollow body having an innersurface area to be coated of 0.85 m² would accordingly have to be coatedwith a power of approximately 1500 watts.

Surprisingly, the bias voltage established under these conditions is inthe region of 0 V, to be precise on all the leads. Moreover, this valueis independent of whether or not the hollow body to be coated is inelectrically conductive contact with the radio-frequency electrode.

The gas feed is regulated in a gas-specific manner and adjusted in arange of 0-90 sccm depending on the object and desired layer properties.It is preferably provided here that the quantity of reaction gas to beintroduced for the coating is 0.1-10 sccm of reaction gas per 10 cm² ofinner surface area to be coated.

The unit sccm denotes standard cubic centimetre, i.e. the volume of thegas to be introduced in cubic centimetres per minute (volume perminute). A valve with a mass flow controller is used for the adjustment.At a given pressure of the gas supply line, therefore, the opening stateof the valve governs the inflowing volume per minute.

In the case of hydrocarbon gases it holds true that the layer becomesall the harder, the more gas is used since the proportion of availablecarbon atoms increases.

In the case of silane gases, by contrast, it holds true that the ratioof the silane gas to oxygen determines the hardness of the layer. Forhard coatings, the ratio is e.g. 100 sccm HMDSO (hexamethyldisiloxane)to 400 sccm oxygen. By contrast, a reduction of the oxygen proportionleads to softer layers.

Particularly preferably, the quantity of the reaction gas to beintroduced is 0.5-5 sccm of reaction gas per 10 cm² of inner surfacearea to be coated.

Furthermore, it is preferably provided that the reaction gas is dopedwith one or more gases containing Si, N, F, B, O, Ag, Cu, V or Ti. Thesedopants can contribute to having a targeted influence on the propertiesof the coating applied. Thus, e.g. the doping of the reaction gas with agas containing Si (e.g. hexamethyldisiloxane) leads to a reduction offriction even under moist conditions and also to a higher thermalstability. A doping with N, F, B or O influences the surface tension,the wettability and the hardness of the coating. A doping with metalscontributes to influencing the conductivity of the coating, while adoping with Ag, Cu, V or Ti influences the biological behaviour of thecoating, in particular the biocompatibility which is hugely importante.g. for implants.

Layer growth rates of up to 4 μm/h and layer thicknesses of up to 7 μmare achieved with the method according to the invention.

The invention furthermore provides a hollow body having an innersurface, characterized in that the latter was treated by a methodaccording to any of the preceding claims in such a way that a materialremoval was performed on the inner surface and/or the latter wasprovided with a coating. The coating can be, as mentioned above, e.g. aDLC, TiOx or SiO₂ coating.

Particularly preferably, said hollow body is a hollow body selected fromthe group containing vessels, bottles, containers, cannulas, hollowneedles, syringes, inner walls of cylinder or piston bores in internalcombustion engines, inner sides of bearings, in particular ball orrolling bearings.

The hollow bodies mentioned can comprise non-metallic materials, inparticular, since the hollow body—in contrast to the description in DE3821815—does not function as an electrode. This opens up newpossibilities in lightweight construction. Thus, it is possible, forexample, to produce a highly loaded metallic workpiece—thus e.g. anengine block of an internal combustion engine—from a plastic and to coatthe inner walls of the cylinder bores with a surface having a highloading capability in a manner according to the invention.

The following advantages, inter alia, can be achieved with the methodaccording to the invention:

a) improved cleaning of three-dimensional hollow bodies, in particularbulk containers, in conjunction with a reduced outlay;b) improved corrosion protection of the coated surfaces;c) no diffusion of a substrate situated in the hollow body into theinner surface layer of the hollow body;d) reduction of the coefficient of friction of the inner surface; ande) improved heat dissipation.

The invention furthermore provides a device for carrying out a methodaccording to any of the preceding claims.

The present invention is elucidated in greater detail by the figuresshown and discussed below. It should be taken into account here that thefigures are only descriptive in nature and not intended to restrict theinvention in any form.

FIG. 1 shows a section through a vacuum chamber 10 according to theinvention in frontal view with a radio-frequency electrode 11 arrangedat the bottom of the chamber, a hollow body 12 to be coated on the innerside and having an opening 13, said hollow body being arranged by way ofa mount 14 on the radio-frequency electrode.

The radio-frequency electrode 11 in the interior of the vacuum chamber10 has three leads 15 via which radio-frequency voltages generated by aradio-frequency generator (RF generator) 16 are fed into theradio-frequency electrode 11. By means of a regulable matchbox 17, as itis called, connected between the RF generator 16 and the radio-frequencyelectrode 11, the individual leads to the radio-frequency electrode 11can be adjusted separately with the aid of trimming potentiometers inorder to generate a homogeneous alternating field having uniformly highfield strengths in the entire chamber.

FIG. 2 shows the same vacuum chamber 20 in a lateral sectional view,with the radio-frequency electrode 21, the hollow body 22 to be coatedon the inner side in plan view with an opening 23, and also the mount24, which is not electrically conductive. The hollow body is a bulkcontainer in the example shown. A gas lance 25 is inserted into thehollow body through the opening 23 of the hollow body, said gas lancehaving a terminal nozzle 26 having a diameter of 0.6 mm at its distalend. The gas lance is connected to a gas supply (not illustrated) via ahose and is guided via a height-adjustable mount 27, by means of whichit can be ensured that the gas lance can be positioned in the hollowbody 22 in accordance with the dimensioning in the main claim. For thispurpose, the mount is arranged on a carrier 28 in height-adjustablefashion.

The radio-frequency electrode 21 in the interior of the vacuum chamber20 has three leads 29 via which radio-frequency voltages generated by aradio-frequency generator (RF generator) 30 are fed into theradio-frequency electrode 21. The individual leads to theradio-frequency electrode 21 can be adjusted separately via a regulablematchbox (not illustrated) connected between the RF generator 16 and theradio-frequency electrode 11.

FIG. 3 again shows a vacuum chamber 30 in a lateral sectional view, witha radio-frequency electrode 31, a hollow body 32 to be coated on theinner side and arranged upright in plan view with an opening 33, throughwhich a gas lance 34 is introduced into the hollow body. In the exampleshown, the hollow body is a bulk container composed of high-grade steel.Thus, in contrast to the embodiment shown in FIG. 2, the hollow body iselectrically conductively connected to the radio-frequency electrode 31,and therefore likewise functions as an electrode.

FIG. 4 shows the same vacuum chamber 40 as FIG. 2, with theradio-frequency electrode 41, the hollow body 42 to be coated on theinner side in plan view with an opening 43, through which a gas lance 25is introduced into the hollow body. An electromagnetic alternating fieldis set [values, three leads, very homogeneous field] at theradio-frequency electrode and gas flows into the hollow body through thegas lance. On account of the electromagnetic interactions, the gasmolecules that emerge are accelerated and a spherical plasma 45 isformed, which is also referred to as a cavity plasma since itessentially remains within the hollow body and does not pass into theactual vacuum chamber 40. The coating effects described above areestablished here on account of the plasma. On account of the via thesuction-extraction connector 46, the outflowing gas or plasma is suckedin the direction of the opening 43.

FIG. 5 shows a coated bulk container 50 in cross section with a wall 51and the coating 52. The bulk container has a depression 53 in the regionof its bottom. Moreover, the gas lance shown in the previous figures andthe spherical plasma that is formed are illustrated schematically. Itcan be discerned that the coating applied on account of the effects ofthe spherical plasma has a greater thickness particularly in the regionof the exit opening of the gas lance than in the edge regions of thecontainer bottom or on the inner walls of the container. The thicknessof the coating is greatly exaggerated in the illustration; in practice,it varies in the range of between 50 nm and 20 μm.

When the bottom of the container is viewed directly by an observer, thisthickness gradient is discernible by virtue of the iridescent colourcaused by interferences with the waves of the visible light spectrum(350-800 nm).

FIG. 6 shows the coating process under way on a horizontally arrangedbulk container. For this purpose a photograph was taken through theporthole of the chamber in the direction of the opening of the bulkcontainer. It can be discerned that the plasma formed burns only in theinterior of the container, and not for instance in the entire chamber asknown in devices from the prior art. The cavity plasma discussed aboveis involved here.

FIG. 7 shows the coating process under way on a vertically arranged bulkcontainer. For this purpose a photograph was taken through the portholeof the chamber in the direction of the opening of the bulk container.Here, too, it can be discerned that the plasma formed burns only in theinterior of the container, and not for instance in the entire chamber asknown in devices from the prior art. The cavity plasma discussed aboveis involved here.

FIG. 8 shows a bulk container coated by the method according to theinvention in frontal view. The container is still arranged in thecoating chamber; the non-electrically conductive mount can be discernedin the lower region. In particular, the depression already discussed canbe discerned in the region of the bottom of the container. It isfurthermore readily discernible that the container has been coated witha DLC coating in the inner region, while the outer side of the containerhas not been coated (evident from the metallically lustrous surfacecomposed of high-grade steel).

FIG. 9 shows the bottom of a bulk container coated by the methodaccording to the invention in frontal view. Here, too, the depressionalready discussed can be discerned in the region of the bottom of thecontainer. Here, too, on the basis of the different brightnesses it isonce again readily discernible that the container has been coated with aDLC coating in the inner region, while the outer side of the containerhas not been coated (evident from the metallically lustrous surfacecomposed of high-grade steel).

FIG. 10 shows the transition region between the bottom and the innerwall of a coated bulk container. In this case, it is in particularreadily discernible that a welding seam arranged in the transitionregion has likewise been coated well.

1. Method for plasma-assisted chemical vapour deposition for coating ormaterial removal on the inner wall of a hollow body, in particularcomposed of a non-metallic material, having a cross-sectional area, alongitudinal extent and at least one opening, comprising the followingsteps: introducing the hollow body to be coated on its inner side into avacuum chamber with an earthed inner side, a large-area radio-frequencyelectrode being arranged in the interior of the vacuum chamber,positioning the hollow body in the centre of the vacuum chamber, itbeing necessary to comply with a minimum distance of 15 cm on all sidesbetween the outer wall of the hollow body and the inner wall of thevacuum chamber, introducing a gas lance comprising a tube having aninternal diameter of 0.001-10 mm, a maximum external diameter of 12 mmand a terminal nozzle having a terminal opening diameter of 0.002-6 mmthrough the opening into the hollow body, the gas lance being connectedto a gas feed unit via a non-electrically conductive gas line and, inparticular, not being earthed or being in electrically conductivecontact with the radio-frequency electrode, positioning the gas lance inthe hollow body in such a way that the gas lance is positioned centrallyrelative to the cross section of the hollow body and the nozzle of thegas lance, relative to the longitudinal extent of the hollow body, isarranged in the region of the transition from the second longitudinalthird to the third longitudinal third, measured from the opening of thehollow body, closing the vacuum chamber and evacuating the latter to aresidual pressure of 0.001-20 pascals, introducing an inert working gasand also one or a plurality of reaction gases via the gas feed unit andthe gas lance into the hollow body, and igniting a cavity plasma to forma plasma cloud arranged at the tip of the gas lance, by applying anelectric radio-frequency field to the RF electrode.
 2. Method accordingto claim 1, characterized in that the radio-frequency electrode in theinterior of the vacuum chamber has at least two leads via whichradio-frequency voltages can be fed into the radio-frequency electrode.3. Method according to claim 1, characterized in that the individualleads to the radio-frequency electrode are adjusted separately in such away that a homogeneous alternating field having uniformly high fieldstrengths can be generated in the entire chamber.
 4. Method according toclaim 1, characterized in that said hollow body has an opening whosenarrowest diameter is narrower than the narrowest diameter of the innerspace of the hollow body.
 5. Method according to claim 1, characterizedin that said hollow body has an internal volume >0.1 ccm³ and <1,000,000ccm³.
 6. Method according to claim 1, characterized in that the workinggas is a gas selected from the group containing argon, helium, hydrogen,oxygen or a different gas.
 7. Method according to claim 1, characterizedin that the reaction gas is a gas selected from the group containingoxygen.
 8. Method according to claim 1, characterized in that thereaction gas is a gas selected from the group containing hydrocarbongases such as methane, ethane, ethene, ethyne, propane or silane gasessuch as tetramethylsilane or hexamethyldisiloxane.
 9. Method accordingto claim 1, characterized in that the plasma is ignited by applying a DCvoltage radio-frequency field with the following parameters: frequency:10 kHz-100 GHz electrical power: 500-5000 W
 10. Method according toclaim 1, characterized in that the amount of reaction gas to beintroduced for the coating is 0.1-10 scm³ of reaction gas per 10 cm² ofinner surface to be coated.
 11. Method according claim 1, characterizedin that the reaction gas is doped with one or more gases containing Si,N, F, B, O, Ag, Cu, V or Ti.
 12. Hollow body having an inner surface,characterized in that the latter was treated by a method according toany of the preceding claims in such a way that a material removal wasperformed on the inner surface and/or the latter was provided with acoating.
 13. Hollow body according to claim 12, characterized in thatsaid hollow body is a hollow body selected from the group containingvessels, bottles, containers, cannulas, hollow needles, syringes, innerwalls of cylinder bores in internal combustion engines.
 14. Device forcarrying out a method according to claim 1, comprising a vacuum chamberwith a radio-frequency electrode arranged at the bottom of the chamber,and with a mount for a hollow body to be coated on the inner side, a gaslance comprising a tube having an internal diameter of 0.001-10 mm, amaximum external diameter of 12 mm, and a terminal nozzle having aterminal opening diameter of 0.002-4 mm, which is connected to a gasfeed unit via a non-electrically conductive line, and aheight-adjustable mount that can be used to ensure that the gas lancecan be positioned in the hollow body in such a way that the gas lance ispositioned centrally relative to the cross section of the hollow bodyand the nozzle of the gas lance, relative to the longitudinal extent ofthe hollow body, is arranged in the region of the transition from thesecond longitudinal third to the third longitudinal third, measured fromthe opening of the hollow body.
 15. Device according to claim 14,characterized in that the radio-frequency electrode in the interior ofthe vacuum chamber has at least three leads via which radio-frequencyvoltages can be fed into the radio-frequency electrode.
 16. Deviceaccording to claim 14, characterized in that the individual leads to theradio-frequency electrode can be adjusted separately in such a way thata homogeneous alternating field having uniformly high field strengthscan be generated in the entire chamber.
 17. Method according to claim 2,characterized in that the individual leads to the radio-frequencyelectrode are adjusted separately in such a way that a homogeneousalternating field having uniformly high field strengths can be generatedin the entire chamber.
 18. Method according to claim 2, characterized inthat said hollow body has an opening whose narrowest diameter isnarrower than the narrowest diameter of the inner space of the hollowbody.
 19. Method according to claim 2, characterized in that said hollowbody has an opening whose narrowest diameter is narrower than thenarrowest diameter of the inner space of the hollow body.
 20. Deviceaccording to claim 15, characterized in that the individual leads to theradio-frequency electrode can be adjusted separately in such a way thata homogeneous alternating field having uniformly high field strengthscan be generated in the entire chamber.